How to turn silica into agates

A Lake Superior agate ground into a sphere.
Photo by Fluorite

Of all the gems in the world agates are one of the most complicated even though they are formed from silica one of the most abundant compounds on earth.  According to Peter Heaney of Penn State University Agates are really complicated stuff whose composition has remained a mystery for many years.  It is only with the application of modern high-tech methods has the makeup of this really complicated mineral been understood.

Agate is composed of what is termed crypto-crystalline silica, or silicon dioxide (SiO₂) that forms long thin fibers of silica that is more a polymer then a crystalline substance.  The real secret of forming an agate Heaney feels about how silica dissolves in water his research came from a need to discover how agate was being deposited in the pipes of geothermal power plants.  Silica kept coming out of solution and was clogging the plants with agate.  His problem was how to keep the dissolved silica from sticking to the insides of the pipes.  The question really was; how do you stop an agate from growing.

A slab of blue agate with a quartz crystal center.
Photo by Tomomarusan 

Because of a line of research I undertook on the formation of geopolymers that makes use of sodium silicate, sodium hydroxide and some sort of pozzalan material to form a similar compound to agates that is used as a green alternative to concrete.  This line of research gives me a useful insight on the formation of agates.  The hot water found deep in the earth contains a lot of dissolved silica that is brought to the surface naturally by hot springs and geysers.

Most agates are found in cavities in basalt or similar magma where it fills the cavities.  Other places where agate forms is in places where it replaces other minerals or fossils.  Petrified wood is one example in other cases it can be petrified bone or even coral as in Florida.   

One of the many forms agate rough can assume.
Photo by Rob Lavinsky

According to the theory an agate tarts with a cavity in a rock with the best being found in basalt. All volcanic rocks that get up to the surface where they harden contain a lot of water and carbon dioxide that will bubble up just like seltzer water. The rocks are full of holes similar to the pipes in the geothermal generating plant. In this case however it is these holes in the rock that fill with the silica solution that become agates. The silica bearing water does not all enter the cavity at the same time causing bands to be formed in the agate. Not only does the silica bearing solution form thee agate and also bears other minerals principally iron or manganese oxide that causes the different colors that are seen in the agate. These solutions percolate through the rock, and the minerals they contain begin to crystallize.

A Lake Superior agate note the banding.
Photo by Iowagateman.

By studying agates with transmission electron microscopy, and by x-ray diffraction and has been found that about 90% of an agate is quartz however there is another mineral and has the same composition but a different structure that is called mogenite. This mineral is more like carbon that can crystallize in either graphite or diamonds. 90% of an agate is quartz the remaining 10% is moganite that is an important factor in the formation of agates.

What an agate is examined under light microscope it displays a layer of fibers crystals that nucleated on the wall and radiate inward into the cavity just like the spokes of a bicycle wheel. Usually the first layer inside the cavity is a very fine material called chalcedony that is a mixture of quartz and moganite. Next in the sequence is a coarse band of quartz crystals that is pure quartz with no moganite.

One of the first questions asked about a maggot is why do we see these two different layers? They are both the same, silicon dioxide, but what is changing the crystal structure of them?

Another curious thing about agate fibers is they twist in growing a helical fashion.

To compound the mystery is the crystallographic direction in which the fibers grow with in bands that favors growing perpendicular to the normal growth direction.

Theoretically what happens is this that causes the characteristic banding seen in an agate with the colors coming from trace elements in the solution like iron and manganese. The agate itself is like a Russian doll having a hierarchical layering very similar to some kind of life form. This may be caused because chemically silicon is very similar to carbon.

Suppose the silica bearing water flowing into the cavity was only a little bit polymerized its precipitates onto the wall. If the concentration of silica gets high enough that polymerizes in short but repeat units of 5 to 10 molecules. It has been theorized that these polymerized molecules get pulled out of solution very quickly and become crystallized very rapidly. Because this happened so quickly any mistakes are made in the crystal structure causing weird minerals to form like moganite.

Under normal conditions all you will see are quartz crystals, but this is the process that proceed slowly. Many times this causes the center of the agate to appear as a geode line with quartz crystals and the rim of the cavity to be deposited as chalcedony. Although the deposits of moganite can proceed rapidly the quartz crystals themselves are deposited quite slowly. If one were to take a glass of water saturated with silica it would take two years before even the tiniest quartz crystals were deposited.

There are also channels that are located between the crystal fibers that work by attraction to pull water into the center of the cavity; thus you have a continuous supply of water feeding into the system that contains silica. The silica will polymerize again when it approaches saturation causing a layer of moganite to crystallize rapidly inside the cavity. This is quite an agate that has its banding pattern. However, this has not been proven experimentally.

One of the things we are certain of whatever is that agates form close to the surface of the earth at low pressures and temperatures where they are found in volcanic rocks, but also as replacements and many other environments. Agates are very resistant to erosion and are often found as just a roundish lump with a rough coating just laying on the surface of the ground waiting to be picked up, cut and polished that can be used in a piece of jewelry.

Gemstone Occurrences in New Jersey

A ruby crystal from Tanzania similar to those found in New Jersey.

Like many of the states of the Northeast New Jersey contains some of the best localities for finding gems in the Eastern United States.  Agate is found throughout the traprock quarries of the state as well as quartz gemstones including amethyst that occurs as cavities in the stone.  Many marine fossils are also found in the state including many fossils of shark teeth. Most of these fossils are found in the Kirkwood formation (Eocene and Miocene) in Monmouth County.

One of the most interesting stones found in the state are so-called Cape May Diamonds that in reality are waterworn quartz crystals that are found on the beaches of Cape May Point along the Delaware River in southern New Jersey.  These stones are the most abundant at Higbee and Sunset beaches in Cape May.  These stones can range in size from the finest sand to one whopper that weighed 3 pounds 14 ounces that is known as the Cape May Diamond that was found in New Castle, Delaware in 1866.

Rubies are rarely found in the white marble at the mining district in Franklin where they are found associated with the zinc mines in Franklin and Sparta.  Many of these stones are small, but are well formed.  The rubies fluoresces red, but the marble matrix doesn’t. Many minerals found on the mine dumps around the area are various minerals of zinc that also fluoresce in brilliant colors under a fluorescent light with some of them even considered gemstones.  Another mineral that is found associated with the zinc minerals is rhodonite a silicate of manganese that is often cut as cabochons.

Bloodstone or heliotrope is found in sparing amounts in the Eastern United States north from Florida with exposures in New Jersey.  This is a member of the quartz family of gemstones that is dark green in color with bright red spots of hematite.  This stone is often cut and used in inexpensive jewelry.

A polished slab of petrified wood.
Photo by Daniel Schwen

In addition to agate the traprock quarries in Passaic County also contain many zeolite minerals as well as datolite and prehnite in large quantities. In many places the traprock quarries have cavities lined with amethyst crystals.  In some areas in eastern New Jersey you can find jasper and petrified wood.

In recent years one of the most valuable gems in North America has been overlooked by collectors is the freshwater pearl found in many of the mollusks in the states many rivers.  The pearl was looked avidly for in the 19^th Century, but for a period of inactivity caused by over fishing the mollusks  caused this industry to decline.   

How Diamonds are Formed

This shows the Earth's mantle where diamonds are formed.
Courtesy of the Debeers Group.

A vast amount of knowledge has been developed since the 1980s about how diamonds were formed.  It has been through the inclusions found in diamonds that much of the information about how they were formed and their origin was discovered,  Inclusions are all natural materials that are found in diamonds including some that are gaseous, liquid and solid inclusions.  Although theoretically it is possible to form diamonds by squeezing a lump of coal in nature that hasn’t happened although in the lab this had been done under conditions of extreme heat and pressure in giant hydraulic presses,

Some of the diamonds that have been found in meteorites that are more then 4.25 billion years old far exceeding the age of life on Earth.  However natural diamonds on earth were formed at least 990 million years ago.  One of the discoveries made since the 1980s shows that diamond is formed whenever you burn acetylene gas that forms as a coating on metal that has been exposed to burning acetylene that is often deposited during the course of oxy-acetylene welding.  This type of diamond coating has since been made use of industrially for imparting a hard coating onto metals.

How a diamond bearing pipe is formed.
Courtesy of the Debeers Group 

It takes extreme pressure that ranges from 40 to 50 thousand times higher then the pressure found at the earth’s surface of 14,7 pounds or 1,000 millimeters that is termed one bar.  This is the pressure encountered at depths ranging from 125 to 200 kilometers below the surface   Most diamonds are formed from kerogene that was deposited in vast quantities on earth during its formation.  There are other diamonds that were formed from carbonaceous material like fossil coral or shells that have been subducted deep into the earth’s crust where they were also exposed to extreme heat and pressure.  A few diamonds were even formed at depths ranging from 300 to 400 kilometers deep or even further, but these diamonds are extremely rare.  Most diamonds were formed at temperatures that exceeded 900⁰ to 1,300^o Celsius.

Formation and Emplacement of a Kimberlite Pipe

At about 125 kilometers beneath the surface of the earth the pressure of the magma cracks the surrounding rock until the cracks reach the earth’s surface.  Much of this cracking is due to contained carbon dioxide in the magma that causes the magma to bubble and expand.  This is caused either by heat from below or reduced pressure as the magma column reaches towards the surface.  It is this expansion that eventually causes the magma to erupt explosively at the surface.

An eruption like this has never been observed by mankind because it has been millions of years since one occurred on earth predating mankind.  It has been theorized that an eruption of this type rather then being hot like most volcanic eruptions are; it is emplaced under freezing temperatures because of the rapid expansion of the volcanic gases.  In the final few kilometers these gases rise at the speed of sound where they form a pipe following the path of least resistance through the surrounding rock finally reaching the surface.

It is because of this rapid ascent of magma that allows the diamonds to reach the surface of the earth rather then being converted into graphite a far more stable substance at the earth’s surface.  The pressure him pardoned by dissolved gases in the magma eject the diamonds they don't have time to be degraded into graphite.  The diamonds are actually xenoliths, particles caught up in the magma train as foreign objects.  Once the volcanic cone cools it starts too erode releasing the diamonds into the soil. The diamonds that are not eroded state trapped in a kimberlite or lamproite where they are mined.

Different Types of Diamond Deposits

How diamonds are mined.
Courtesy of the Debeers Group.

Diamonds are found in two different types of deposits of which primary deposits come first.  These are deposits found in kimberlite, lamproite or komatiitef pipes and are mined lake any other mineral found in hard rock.  Secondary diamonds are found in river gravel deposits where they are concentrated after traveling for some distance from their primary source.  The gravel found in the Orange River in South Africa is an important source of diamonds.

Often overlooked by many is a third source of diamonds found in a rock type called conglomerate that are really fossilized river gravel.  One important formation like this is found near Marathon, Ontario that not only carries diamonds, but rubies and sapphires as well..

Appalachian Diamonds

A diamond crystal in matrix
Photo by Rob Lavinsky

Look out, alluvial diamonds have been found scattered along the backbone of the Appalachians for years making it likely that sometime in the future a lucky prospector is likely to find a kimberlite or lamproite the two types of rock where diamonds are apt to be formed.  Diamonds have been found in Virginia, West Virginia, North and South Carolina, eastern Kentucky and Tennessee, Georgia and Alabama.  All of these states are below the area that was affected by the continental glaciers of the past million years making it likely that their host rocks are in place.

At the time of its discovery in April 1928 the famous “Punch” Jones at 34.43 carets was the second largest diamond ever found in the United States.  The Uncle Sam diamond was found in 1924 at Murfreesboro Arkansas in the Prairie Pipe Mine weighing at the time of its discovery 40.23 carets is the largest diamond ever found in North America.  The Punch Jones diamond is presumed to have washed Rich Creek from neighboring Virginia.   There are five sites in or near to Virginia where diamonds have been found one in kimberlite and one in peridotite; it is presumed all of these diamonds came from similar rock types.

Brilliant Cut Diamonds

North Carolina produced 13 diamonds in the mid-1800s.  The North Carolina Geological Survey has recently undertaken studies of lamproites in the Charlotte area to better understand where these diamonds originated and to discover clues as to their origin for other exploration projects in the Piedmont.  The other Southern States produced one or two additional diamonds.  The lack of glaciation in all these states suggests a local origin for theses diamonds.  Lamproites and kimberlites have been discovered in the immediate areas where most of these diamonds have been found, but none have been found since WW II when gold mining operations ceased in the southern Appalachians.

Resumption of placer mining operations make the discovery of diamonds likely.  Most of the diamonds discovered in the past were from placer mines with a few exceptions.  The Punch Jones diamond is one of these exceptions it was discovered when Punch Jones and his father were playing horseshoes.  Although this diamond was discovered in 1928 it took until 1942 before its true identity was established.

Fortunately diamonds have some distinct minerals associated with them making it possible to find diamond deposits by tracing the diamonds and other associated minerals to their source.  These minerals include type “G” pyrope garnets, chrome diopside, phlogophite mica and serpentine.  The mineral illmanite is frequently found associated with diamonds too.  Tracing these minerals is often done with soil samples taken below the “A” horizon that contains organic matter.  

Gemstone Occurrences in New York:

Herkimer Diamonds from the Ace of Diamonds Mine.
Photo by Rob Lavinsky

You could almost write a book just about gemstone occurrences in Manhattan that has produced beryl, chrysoberyl and some huge garnets.  These are all gems that have been produced from excavations and the rock removed from them.  Most of this rock now resides along the bank of the Hudson River in uptown Manhattan where you can readily get at the excavation spoils from Inwood Park.  The entire southeast part of the mainland has the same kind of gemstones as Connecticut with whom it shares a common border.

Saratoga County has produced beryl and rose quartz from the pegmatites found in the Adirondacks.  Almandine garnets have been produced from a deposit on Gore Mountain in the town of North Creek..  These garnets are embedded in a matrix of gabbro with some of the crystals reaching more then a meter in diameter.  The Barton Mining Company produces the garnets for making abrasives, but sufficient amounts of cuttable material is produced to satisfy the needs of the Rock Hound trade.

In the middle of a city park in Syracuse is a kimberlite pipe that may be diamondiferous or not depending upon who is telling you about it.  This is a swarm of pipes that extend from a coal mine in Pennsylvania to the Monteregian Hills of Quebec. Many of these kimberlite dikes are clustered around Ithaca, but the narrow swarm of kimberlites can be found crossing the state from the Pennsylvania border to Ogdensburg on the St. Lawrence River. None of the NY kimberlites are known to contain diamonds.  Diamonds have been found in the glacial deposits that were brought down from the diamond deposits of Ontario.

Kimberlite
Photo by Sub Arctic Mike

The original discovery of a kimberlite dike in New York occurred in Syracuse in 1837, but was mistaken for serpentine; it wasn’t until 1887 that rocks of this nature were properly identified at Kimberly South Africa that their true nature was discovered.  The kimberlite dikes in New York although they don’t contain diamonds because the magma came from too close to the surface of the earth do contain some other minerals that are considered gemstones providing they are large enough including olivine and spinel.

An interesting deposit of rubies and sapphires are found in the marble deposits of Orange County.  The quantity of stones that have been found in sparse, but the deposit extends into Sussex County New Jersey. These gems were found somewhere between Monroe and Southfield in Orange County. According to the report these gems were less then two carets in weight, and the deposit was sparse. This was a deposit that was worked secretly by two people for some years who took its locality to the grave with them.

When the zinc mine at Edwards was being actively mined they discovered a mass of lazulite weighing more then twenty pounds in the process of mining zinc ore.  This mass was cut up into cabochons and made into jewelry.  Lazulite is one of the components of the semi-precious stone lapis lazuli. 

A stone that is readily identified with New York State is quartz whether it is in the form of amethyst in the trap ridges along the Hudson River in the Haverstraw vicinity, at the old lead mine in Ellenville or in the Little Falls formation crossing the state from west to east where it ends at diamond cliff in Hague on Lake George.  This is the home of the world famous Herkimer Diamond that in reality is a doubly terminated quartz crystal found in cavities in the silicified dolomite of the Little Falls formation.  Various varieties of quartz (silica) gemstones are found throughout New York.  Albany County is noted for its flint and jasper that is found in sand and gravel deposits.  There was even a flint deposit overlooking the Hudson Valley that was worked by the Indians in prehistoric times for flint to make their weapons.

Gemstone Occurrences in Connecticut

Almandine garnet, Connecticut's state stone.
Photo by Rob Lavinsky

Connecticut is amply endowed with gemstones ranging from andalusite to zircons.  The stones found in Connecticut are found in a variety of terranes although the primary one is pegmatites, but they are also found in the trap ridges of the Connecticut River Valley.  Even more are found as glacial float if you want to use that method of finding gems.

Just in the town where I live, Barkhamsted there are several different verities of gemstones including almandine garnets, beryl, corundum, kyanite, quartz, soapstone, zircon and the usual rock forming minerals.

Connecticut is blessed with many gem bearing pegmatites one series starts in Glastonbury and Crosses the Connecticut River at Middletown and extends almost to the coast of Long Island Sound.  The other belt of pegmatites extends diagonally across the western portion of the state from Hartland to Ridgefield.  Both of these belts contain many of the gemstones found in LCT type pegmatites.  Many of them are Dana localities and one of them, the Maryall Mine is considered to be one of the most important gemstone mines in the United States.  This mine is famous worldwide for its crystals of aquamarine and golden beryl.  It also contains some bright red garnets that can weigh up to several pounds.

Beryl crystals on Feldspar.
Photo by Rob Lavinsky

The trap ridges of the Connecticut River Valley and in Woodbury contain specimens of amethyst, datolite and prehnite that are found in vugs in the trap rock.  Some of these occurrences are also Dana localities. Many of them are also world famous.  Connecticut has over 600 recognized mines that are now abandoned scattered mainly across the western half of the state, but there is still plenty of gems to be found in eastern Connecticut.

Microlite-Elbalite and Lepidolite found in the pegmatites of Connecticut.
Photo by Rob Lavinsky

The beaches in eastern Connecticut are littered with moonstones that have eroded from the granite eastern Connecticut is known for.  Another stop you should make is the Connecticut Mining Museum that is part of the Sloan-Stanley Museum in Kent where many of the minerals found in Connecticut are displayed along with lots of information about the mining industry that used to be found in the state.  According to many authorities modern mining was developed in Connecticut then spread out all over the world.

Gemstone Occurrences in Rhode Island

This is an amethyst scepter like those found at Diamond Hill
Photo by Rob Lavinsky

There is a greenish mica schist found in Warwick that has embedded in it red almandine garnets. In East Warwick you can find purple almandine garnets embedded in a silvery mica schist. Jamestown has staurolite crystals embedded in phyllite. Garnet-staurolite bearing schist can also be found under the Jamestown Bridge,

A discovery of beryl was recently reported in the magazine Rock and Gem on the western side of Narragansett Bay in the vicinity of the Jamestown Bridge in the pegmatites found in the area. It was also found at the end of the Cliffwalk in Newport.  The largest crystal found in Newport was about 6 cm in length and 1 cm in diameter along with several other smaller specimens.

There have been moonstones reported on the beaches of Rhode Island that continue into eastern Connecticut.  One of the beaches in Rhode Island is actually called Moonstone Beach.

The state stone is bowenite a variety of serpentine of the sub variety antigorite that has often been classified as a semi-precious stone that is cut into polished slabs or cabochons.  The Maoris of New Zealand have also used bowenite to make axes.  This stone is associated with limestone and was originally found in Lincoln.

Amethyst has been from several localities in Rhode Island notably however from Diamond Hill in Ashaway, Hopkinton in Washington County.  This mineral is accompanied by several other varieties of quarts.  At times the amethyst from this locality is found as rare scepter specimens.

A variety of Jasper veined with chalcedony is found in several localities in Cumberland, Providence County.  Chalcedony has also been found at the Conklin Quarry in Lincoln and another locality is Diamond Hill in Ashaway.